As is known in the art, a switchable transmit/receive (T/R) module is used in many radar system applications to direct a radar signal produced by a radar transmitter and fed to an antenna during a transmit mode and to direct radar returns received by the antenna to a radar receiver during a subsequent receive mode selectively in response to electronic logic control signals fed to switches used in the T/R module. More particularly, referring to FIG. 1, a radar system is shown having beam forming apparatus to form beams of electromagnetic radiation. The shape of the beam is related to the phase and amplitude distributions provided to signals received or transmitted across an aperture of the apparatus. For example, in a phased array antenna, as shown in FIG. 1, the aperture includes a plurality of antenna elements. Each one of the antenna elements is coupled to a feed network through a corresponding one of a plurality of switchable transmit/receive (T/R) modules. Each one of the plurality of switchable transmit/receive (T/R) modules includes a variable phase shifter and variable gain/attenuation element. The feed structure may be a corporate feed or may be through illuminations as in a space fed phased array system. In any event, the modules are controlled by signals from a beam steering computer to provide a collimated and directed beam of radiation. For example, for a broadside (i.e., boresight) beam the phase shift of the signals emanating from each antenna element is zero relative to some arbitrary reference. If the phase shift from element to element are in-phase, the direction of the main radiation lobe is shifted from broadside accordingly.
Each one of the switchable transmit/receive (T/R) module includes: set of three T/R switches controlled by logic signals produced a CLC control interface (or control logic) in response to transmit enable and receive enable signals produced by a system controller; a digitally controlled attenuator and digitally controlled phase shifter, controlled by signals fed by the beam steering computer, a high power amplifier (HPA); a circulator and a low noise amplifier LNA, arranged as shown. During transmit, RF energy from the radar system is fed to the plurality of antenna elements through the feed network, the switchable transmit/receive (T/R) modules, the high power amplifiers (HPA), and the circulator. On receive, energy received by the antenna elements is fed to the radar system through the circulator, low noise amplifier LNA, the switchable transmit/receive (T/R) module, and feed network.
One such T/R module, a Common Leg Circuit (CLC) wherein both the transmit signal and the return signals pass between an antenna and circulator to the radar system through common gain controlled attenuators and phase shifters is shown in FIGS. 1A and 1B; FIG. 1A showing schematically the position of the three T/R switches, each including a pair of Field Effect Transistors (FETs; FET 1 and FET 2) as shown in the transmit mode and FIG. 1B showing schematically the position of the three T/R switches used in the switchable transmit/receive (T/R) module in the receive mode. The FETs 1 and 2 in each one of the T/R switches is controlled by control signals (logic signals) produced by control logic to be described below, in response to transmit enable and receive enable logic signals produced by the system controller (FIG. 1).
As is also known in the art, in the ability to switch between transmit and receive modes quickly enables higher performance for the radar system. In traditional Gallium Arsenide (GaAs) High Power Amplifier (HPA) based radar applications, the common source FET is used as the HPA. In the transmit mode this HPA FET is shutdown very quickly since its drain voltage decays very fast. More particularly, FIG. 1C is a truth table showing the relationship between the logic signals produced by the control logic in response to the transmit enable and receive enable signals produced by the system controller and FIG. 1D shows timing diagrams for a repetitive sequence of transmit/receive modes of operation. It is first noted that the control logic section includes a pair of NOR gates; NOR gate 1 and NOR gate 2 arranged to provide a “flip/flop” circuit.
Thus, assume that a transmit enable logic 1 signal is applied to NOR gate 1 and that a receive enable logic 0 signal had been fed to NOR gate 2; therefore, the logic 1 fed to NOR gate 1 produces a logic 0 fed to the gates of the three FETs 1 turns the three FETs 1 “off” while the NOR gate 2 produces a logic 1 fed to the gate of the three FETs 2 turning the three FETs 2 “on” so that RF passes from the beam forming network to the antenna, as indicated by the arrow in FIG. 1A.
After the transmit enable logic 1 is removed, or changed from a logic 1 to a logic 0, to wait for a subsequent receive mode, the logic 0 remains on the receive enable, so that NOR gate 1 still produces a logic 0, a logic 1 is still produced by NOR gate 2 and the three FETs 1 continue to stay “off” and the three FETS 2 remain “on”, so the transmit mode is maintained.
When a receive enable logic 1 is subsequently received, it being noted that the transmit enable logic 1 had been removed, or changed from a logic 1 to a logic 0, to wait for a subsequent receive mode, the receive enable logic 1 turns the three FETS 1 “on’ and the three FETs 2 “off” whereby RF received by the antenna passes to the beam forming network as illustrated by the arrow in FIG. 1B. It is noted that the system remains in the receive mode even after the receive enable signal switches from a logic 1 to a logic 0 because the logic 0 on the transmit enable is fed as a logic 1 input to NOR gate 2 which produces a logic 0 on the three FETs 2.
It is also noted that the radar needs the transmit elements to be turned all the way off before the radar system can switch into the receive mode. Due to this, traditional systems Gallium Arsenide (GaAs) FET switches were able to keep a last state logic level sent to the RF routing switches for low logic levels at both Transmit and Receive enable. The transmit path is enabled in the dwell between transmit pulses and receive returns, but the drain voltage on the HPA FETs drops quickly enough that no RF energy is generated by the transmit chain of GaAs power amplifiers.
In Gallium Nitride (GaN) MMIC based radars this becomes difficult because the impedance of the GaN HPA FET causes a slow decay of the drain voltage when the transmit mode is turned off (terminated or shutdown). This slow decay causes a delay before the receive mode can be turned on. However, for GaN based High Power Amplifiers (HPAs) that are used in more modem transmit/receive modules, a bleeder circuit is sometimes used to drain the voltage quickly but is not practical for a CLC.